CAPE CANAVERAL, Fla. – Shuttle Discovery and its crew of seven thundered into orbit yesterday, setting off on the most challenging space-station construction mission yet, one that will pave the way for the arrival of two science labs in a few months.

ELIOT J. SCHECHTER / Getty Images

Space shuttle Discovery headed for the International Space Station on a mission considered the most complex since station assembly began in orbit.

NASA pulled off the on-time launch after determining that a small patch of ice on fuel-tank plumbing posed no danger. Indeed, most if not all of the ice harmlessly shook loose when the booster rockets and engines ignited.

The rain clouds that had been forecast for days stayed away for the late-morning launch.

“We got lucky today. We could have just as easily gotten unlucky,” launch director Mike Leinbach said. “But as I tell my team, there's nothing wrong every now and then with a little good luck.”

With Discovery safely in orbit, NASA looked ahead to all the work awaiting the astronauts once they arrive at the International Space Station tomorrow. It is considered the most complicated mission in the nine years of station assembly in orbit.

During their 1½-week visit to the station, the astronauts must install a live-in compartment that they are bringing along, relocate a giant girder and set of solar wings, extend those solar wings and radiators, and test a thermal tile repair kit.

In all, five spacewalks are planned, which will be the most conducted while a shuttle is docked at the station.

The three space-station residents face even more construction chores after the shuttle leaves, each of them crucial.

NASA's space operations chief, Bill Gerstenmaier, said the two crews face tremendous challenges, but added, “I can't think of a better start to this mission than what we got today.”

In a historic coincidence, both the shuttle and station have women at the helm. Retired Air Force Col. Pamela Melroy is the second woman to command a shuttle, and biochemist Peggy Whitson is the first female skipper of a space station.

At least six pieces of foam insulation came off Discovery's fuel tank during liftoff, but because that occurred after the crucial first two minutes, the debris posed no risk to the shuttle.

“It's preliminary only, but it did look like a clean ascent,” Mission Control informed Melroy.

NASA has paid extra attention to launch debris after the 2003 Columbia disaster. A hole in the wing brought down that shuttle, the result of a strike by a slab of fuel-tank foam.

A much smaller piece of foam broke off a bracket on the fuel tank during the last launch in August, possibly along with some ice, and gouged Endeavour's belly. More changes were made to Discovery's fuel tank to prevent dangerous ice buildup from the super-cold propellants.

Melroy and her crew will use a laser-tipped inspection boom today to check Discovery's vulnerable wings and nose, standard procedure after the Columbia accident.

They will pay particular attention to three of the 44 panels on the leading edges of Discovery's wings that may have cracks just beneath a protective coating. Even though NASA's own safety group wanted to delay the launch, senior managers decided a week ago that wing repairs were unnecessary.

Discovery's primary payload is the Italian-built compartment, about the size of a small bus. Paolo Nespoli, an Italian astronaut making his first spaceflight, is delivering the chamber, named Harmony by schoolchildren who took part in a national competition. About 130 of those youngsters were on hand for Discovery's launch.